Achieving the Striking Colour

"Is there anything special I have to do to fire striker glass?  Can I mix striker and non-striker in the same kiln or piece?"

Strikers generally need a two-hour soak at slumping temperatures, about 660C.  This heat soak helps ensure full development of the colour. If the soak is not long enough, the colour may not achieve the target colour at all, or be paler than anticipated.

The rate of advance to the heat soak is not critical.  But it does need to be the appropriate rate for the thickness and nature of the assembly of glass being fired.

If you were to have too short a heat soak, you can fire again to help mature underdeveloped colours.  This will, of course, change the profile of the finished piece.

Strikers are compatible within their manufacturer’s own range. So, they can be combined in the same piece as any other of the glass in the fusing compatible range.  That means that they can be fired in the same kiln load as non strikers.

The two-hour soak at slumping temperature will not harm the later stages of firing, but it might lead to use of a slightly lower temperature tack fusing than without the long heat soak.  That is because of the heat work put into the glass at the lower temperature.   Only observation will tell you how much less temperature is required.  It may be possible that only a little less time at the forming temperature is required.  Again, only observation will tell you that.

Strikers require a heat soak to mature the final colour.  These striking glasses are compatible with the rest of the fusing range from a single manufacturer. Glass from different manufacturers must be tested for compatibility before combined into a project.

More Information on Citric acid

Vinegar is not recommended for cleaning of glass, and especially not to soak glass in to remove kiln wash or investment materials.

The reasons for avoiding vinegar are that

·        Dilute vinegar - as culinary vinegar is - attacks glass, giving a mild etch to the surface similar to devitrification.  Concentrated vinegar – oddly - does not attack glass as strongly.

·        It is of variable quality – due to uncontrolled strength, various culinary additives, etc.,

·        It has a strong odour, and

·        It takes a long time to work.

Citric Acid

However, there is an acid which works very well to remove investment materials and kiln wash without affecting the glass.  It is the humble citric acid.

Citric acid is a weak organic acid that has the chemical formula C₆H₈O₇. It occurs naturally in citrus fruits, although that is not the best source for cleaning purposes.

More than two million tons of citric acid are manufactured every year. It is used widely as anacidifer, as a flavouring agent and chelating agent. It is the last that is of most interest to kilnformers.

Chemical characteristics

A citrate is a derivative of citric acid. There are many formulations. Two examples are a salt that is named trisodium citrate (also known as sodium citrate); and an ester called triethyl citrate. We are more interested in the first as it is cheap and widely available.

The citrate ion forms complexes with metallic cations.  It forms complexes even with alkali metal cations. This makes citric acid an excellent chelating agent, especially of interest in removing kiln wash and refractory materials from glass.

Chelation 

This is a type of bonding of ions and molecules to metal ions. The agents are usually organic compounds. Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, in MRI scanning, in chemical water treatment to assist in the removal of metals, and in fertilisers, among other things. 

Citric Acid as a Cleaning and chelating agent

Citric acid is an excellent chelating agent, binding metals by making them soluble. Among many cleaning uses are:

    to remove and discourage the build-up of lime scale, from boilers and evaporators. 

    to treat water by chelating the metals in hard water, cleaners produce foam and work better without need for water softening. Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. 

    A solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. 

    Citric acid can be used in shampoo to wash out wax and colouring from the hair. 

    In industry, it is used to dissolve rust from steel and to form a coating on stainless steels to resist corrosion.

Its use in kiln forming is to make use of the chelation properties when dealing with kiln wash and investment material residues.  Aluminium hydrate is the main ingredient of all kiln washes.  When it becomes bound to glass, it is impervious to almost all chemicals.  The chelating property of citric acid enables the bond between the glass and the kiln wash to be broken by incorporating the molecules within its own, making a colloidal solution.  This process is approximately 6 times faster than any vinegar solution and without the odour and etching risks.

A sample of the affected glass followed by 4 hours in citric acid and 24 hours in vinegar.
Credit: Christopher Jeffree

A 5% solution made up with 50gm of granular citric acid in 1 litre of water is all the strength that is required. The affected glass can be soaked in this solution for the time required to complete the chelation without the risk of etching, and without needing ventilation to remove smells.  Unless you are using a lot of cleaner, it is better to make up much smaller amounts as mould can grow on this organic solution.

A 5% solution made up of 50gms citric acid in 500ml of water and 500ml of isopropyl alcohol makes an inexpensive and effective glass cleaner. However, if left for a length of time, it becomes sticky.  Apply the solution, scrub the glass and immediately wipe off the solution.  Then polish the glass dry. The alcohol in the solution makes keeping large quantites possible. 

This post was compiled with the assistance of Wikipedia, Christopher Jeffree and my own experience.

Although this post remains valid, there is another chemical for long soaks to remove mould material or kilnwash.

Citric Acid Cleanser

Christopher Jeffree has kindly outlined the reasons for the effectiveness of citric acid as a cleaner for removing refractory mould residue and acting on kiln wash stuck to glass.  This is his work (with a few personal notes removed).

"Citric acid works well for removing the plaster scale that builds up in vessels used to mix plaster, and it helps to remove traces of investment plaster and kiln wash from glass.  Its metal-chelating properties probably help with dissolution of calcium deposits, but I am less clear why it is so good at removing kiln wash.  The constituents of kiln wash are kaolin and alumina hydrate, neither of which I would expect to be soluble in dilute acids.  Equally, the refractory materials in investment formulae I would expect to be insoluble.  However, kaolin forms layered structures in which flakes, molecular layers, of alumina hydrate and silica interact through hydrogen bonding. It is possible (I am guessing here) that citric acid can disrupt those hydrogen bonds, thereby disaggregating the clay.  All we can say is that empirically, it works.

"I prefer to use citric acid partly because it has a defined composition, but also because it is safe and pleasant to handle – no odour, and comes in the form of easily-dissolved dry crystals like granulated sugar.  Vinegar stinks, and glacial acetic acid is  an aggressive flammable, corrosive liquid with a chokingly acrid smell.

"Calcium sulfate has low solubility, but is not completely insoluble in water - gypsum (calcium sulfate dihydrate) has a solubility of about 2.5g per litre (0.25%)  from 30-100 C. Its solubility is retrograde, meaning that it decreases, rather than increasing, with temperature.  Natural gypsum is an evaporite, a type of rock that often forms by evaporation of lake water in a geological basin with little or no outflow. It can also be produced hydrothermally in hot springs, when water containing sulfuric acid passes through limestone.  

"Calcium citrate is not very soluble either, only in the order of about 0.85g per litre, but the important thing from our point of view is not to get the material into solution but to separate its crystals and make it detach from the glass.

"In other contexts, warm citric acid is used by jewellers and silversmiths as a pickle for dissolving copper oxide (firestain) from silver and gold alloys  after heating / soldering.  It is a safer alternative to the traditional jeweller's pickle of 10% H₂SO₄.

"Citric acid also dissolves rust from iron, without much etching the iron itself, so is good for cleaning rust off tools etc.

"These pictures show a plaster mixing bowl with (presumably) CaSO₄-rich deposit on the surface, cleaned by soaking with 5% citric acid for 4 hours,

and flash from the pate de verre castings with tightly adhering kiln wash, cleaned using 5% citric acid soaked for 4 hours, and vinegar (white wine) soaked for 24 hours.

"I'm not sure about reaction products - I was speculating a lot there, running through hypotheses that I can't support. We don't really have data on the composition of the layers that are stuck to the glass, or a clear idea of why they sometimes stick and sometimes don't (e.g. the differences between transparent and opal glasses in this respect). Maybe this would be a topic to discuss with technical people at Bullseye."

Hope this helps
Best wishes

Chris Jeffree

Subsequent to this work Christopher has done more work and found that Tri-sodium citrate is an even better chemical for cleaning glass of kiln wash and mould material.

Placing of glues

The placing of glues to hold the glass pieces temporarily is important.  Often unsightly black marks appear due to inappropriately placed glues.  Bubbles can form between layers  and even appear to come from underneath the glass for the same reason.  Placing is often more important than the amount of glue used.  Still, the amount used should be the minimum to hold the glass from moving from bench to kiln.

Place glues at edges of the pieces to be secured during movement.  This allows the burn-off of the glue to evaporate without being trapped under the glass.  If you use very runny or diluted glues, the capillary action will draw the required amount of glue under the glass piece to form a secure adhesion.

Glues burn off and leave the glass pieces unsecured long before the glass becomes tacky enough to stick together.  This means that if your stack of glass will not stay in place without glue as you build it, the glass will collapse or move in the kiln.  Glues are only suitable to stabilise the glass pieces while moving to the kiln.

Two recommended glues that burn off cleanly are the Bullseye Glasstac (more fluid) and the Glasstac gel (more viscous)

Kiln Characteristics Investigation

Many people ask about the best kiln to buy.  Sometimes they mean the cheapest, but mostly they mean the best for their favoured processes. To get the best from your proposed kiln, you should be aware of its characteristics and how it fits your proposed kilnforming practice.  There are a range of factors that interact to give the special conditions of your kiln.  They range from the purpose, the materials of construction, the placement of heating elements, how it opens, and its shape.  All these can affect the degree of even heating of the kiln bed or shelf.

Kiln types

There kilns for many purposes. Some of them are powder coating of metals, enameling of metals, vitreous painting of glass, glass forming, ceramics, casting of glass and metals, lehrs for annealing, and furnaces among many others.  

Large powder coating kiln

Large enameling kiln

Jewellery enameling kiln

Electric glass painting kiln with multiple shelves

Example of a sheet glass annealing lehr

For our purposes we are concerned with the glass and ceramics kilns.

Ceramics vs Glass Kilns

In general ceramics kilns are made to lose heat slowly, while glass ones are designed to lose heat relatively quickly.  There are many glass kilns based on ceramic ones.  You should be aware of the differences between kilns designed exclusively for glass and those based on ceramics kiln designs.

Small ceramic kiln

Small glass kiln

Construction Materials 

The materials used in constructing kilns are refractory insulation and a steel structure of a design to hold all the refractory materials together. 

Refractory bricks for glass kilns are light weight and usually designed for temperatures under 1200°C (dense bricks rated much higher are normally used in ceramic kilns). 

Light weight refractory brick

Bricks tend to be used in most glass kilns on the floor as well as the walls (some smaller ones use only refractory fibre).

Small fibre kiln

Kilns derived from ceramics tend to have brick walls and lids.  Most kilns designed for kilnforming have fibre walls and lids.  In the cases of top hat opening kilns, fibre is a necessity to reduce the weight of the lid.

Fibre board and fibre blanket are used widely.  The floor tends to have a floor consisting of steel, fibre board on top and brick on top of the board. Fibre blanket tends to be used on the walls and ceilings of rectangular glass kilns. Oval and circular ones tend to have brick walls and ceilings.  The use of fibre board and blanket walls and ceilings leads to a more rapid cooling than those with brick ones.  This will affect the scheduling of the kiln firings.

The steel used to contain and support the refractory materials is important.  Many kilns use mild steel in sheet form to fill the spaces between the heavier structural support steel.  The higher quality kilns use stainless steel sheet, even though they may use mild steel for structural support.  The stainless steel lasts much longer than mild steel, especially when there is liable to be moisture involved in the kiln processes, such as pate de verre or casting.

Opening Method

This post gives a description of the common methods of opening the kiln.  

The purposes for which you want to use the kiln relate to the firing characteristics needed.

Top opening

Top opening kilns have the advantage of depth, normally with elements around the sides.  This makes them good for casting, but not so good for processes that need observation or manipulation.  The depth is most useful in casting  and deep slumping work, but requires a lot of experimentation to make use of multiple shelves in one firing.

Front opening kilns have the advantage of being able to observe the whole depth of the firing, if you protect yourself from the heat that will be dumped from the kiln.  They often have elements on the sides which is an advantage for drops and melts (when observation is necessary).

Top hat opening kilns are those that have the whole heating chamber hinged at the shelf level.  These are very good for placing of work, as you can work directly above the pieces.  These are one of the best types of kiln for combing or any other manipulation of the glass during the firing. You can also observe by opening the kiln a little during the firing.

A range of top hat and a bell kiln

Bell kilns are those where the whole of the heating chamber lifts above the bed.  These are often equipped with two bases which can be wheeled in turn under the chamber which is lowered before firing.  These tend to be very large kilns.

Small gas fired kiln

Heat source

Most kilns are heated with electrically powered elements, either exposed or in quartz tubes.  The quartz tube contained elements provide more even heating than the exposed ones.  The most even heat is provided in gas fired kilns, although these are generally more expensive and less widely available.

Element Placing  

The location of the heating elements can have a significant influence on the way you fire your glass.

·        Top fired kilns are generally the easiest to use as the glass is most affected by radiant heat.

·        Side fired kilns provide the radiant heat to the edges of the glass first, before the air temperature can begin to affect the surface of the glass.  This means more caution is required in the heat up of the glass.  However, side elements are very useful in drops and casting processes.

·        Some kilns have both top and side heating elements.  This provides flexibility in heating up and in cooling evenly.

·        A few kilns have elements around the sides but below the shelf.  This promotes even cooling of glass from both the top and bottom. It is most useful in dealing with the cooling of thick slabs.

Kiln sizes and shapes

Kiln sizes have an effect on the behaviour of the kiln.  Smaller kilns (depending on the refractory materials) generally heat and cool quicker than large ones.  The mass of a larger kiln takes more energy to heat up and more time to release the heat than smaller ones do.  This will influence the scheduling for different sized kilns.

 

The shape of the interior of the kiln affects the distribution of heat within the chamber.  Rectangular kilns tend to have cooler corners than circular ones (as there are no corners).  Oval kilns tend to give space for longer pieces and reduce the cool corners.

 

The height of the kiln also affects the heat distribution within the kiln.  Taller kilns are cooler at the bottom than the top, even with side elements.  They are especially good for casting and drop processes.  Deeper kilns, even if rectangular, require more energy to complete any given process, because of the distance between the radiating elements and the glass.

Hot and cold spots can be tested for by using this method.  The actual operating temperatures can be tested by the use Orton cones to measure heat work. This depends on the speed used to get to the process temperature.

There are many factors that make up the characteristics of kilns. The main ones are style, construction materials, opening method, shape and depth. These need to be considered in relation to the kind of kilnforming you intend doing, to make the selection optimum for your practice.

More information is available in "Your New kiln" from Etsy shop VerrierStudio: https://www.etsy.com/uk/shop/VerrierStudio

or direct from stephen.richard43@gmail.com

Large Tiles for Kiln Shelves

Pizza stone in use

People frequently wonder if other materials than mullite can be used for kiln shelves.  Mullite is used for its strength and very small expansion, even at high temperatures, as used in ceramics firing.  There are other materials that can be used in kilnforming of glass such as refractory fibre board, and ceramic pizza stones,  the best of which are made from mullite.  This post is about using ceramic floor tiles.

An unglazed floor tile, 11 x 11 inches

Structural Soundness

A major element in obtaining and using a floor tile is how sound it is.  Tapping the tile to determine whether the sound is a low toned ring or a dull thud is important.  There may be invisible cracks within the tile.  A dull thud is an indication that the whole tile has one or more cracks in it, or that it has not been fired high enough to completely vitrify the clay. A low frequency tone indicates there are no cracks and that it has been fired sufficiently high.

Flatness

The first thing you need to do is make sure the ceramic tile is flat and without undulations before using it. To test this, get a straight edge and move it along the tile to look for any slivers of light coming through underneath the straight edge. Any light or variation in the amount indicates depressions that can produce bubbles during the firings. Do this test at least twice at right angles to each other.  Take note of the depressed areas (or even possibly high areas) to know where these uneven areas are to work them out of the tile. 

You can do the above test in the showroom.  Another more accurate means of checking is more difficult to do in a sales area.  Place a line of dark powder, say black glass powder, and with a straight edge held vertical to the shelf, drag the powder across the shelf.  Where there are dark patches is an indication of depressions.  The area and depth can be seen from the spread of the visible powder and to some extent the density of the colour.

Making Shelf Flat

If you buy two of these large tiles, you can rub them together face to face in circular motions. The abrasion marks will show the high spots, with the low spots clear of those marks.  This will indicate the amount of work needed to get the whole surface even.  The smaller the unmarked areas, the less grinding will be required. You can add an abrasive with some water to form a slurry and continue to grind until everything is even. The use of water with the abrasives is important to eliminate dust which might be harmful, and to ease the grinding process.

The above is a manual process.  If you have a large enough flat lap, you can mechanise the flattening process.  Using decreasing grit sizes, you can grind the shelves level with a high degree of smoothness. You do not have to use a grit of less than 200, as the tile structure is even more coarse than that.

If you can't find unglazed floor tiles, you need to look at the back of the tiles.  Many floor tiles have a grid pattern on the back to ensure sufficient adhesive is used.  This makes getting the back, unglazed side flat more difficult or time consuming, because they will need to have the grid ground down to the lower surface.  In this case, it may be that you need to sandblast the glazed side before making sure it is flat.  The sandblasting can make a flat tile uneven by unequal times spend on various parts of the tile, so you have to check after sandblasting for the flatness.

Bubbles on Drop-out Rims

Sometimes people doing dropouts get bubbles or unevenness on the rims of their pieces.  This means that it is not suitable to leave the rim on the piece.  Most times, this does not matter, as you intend to cut the rim away. But if you do want to have a rim these uneven surfaces are unsightly and not suitable for high quality pieces.

One person has indicated that they used a schedule of 250°C per hour to 520°C for a 30-minute soak and then proceeded at 330°C to the top temperature of 710°C.  This is probably too fast a heat up at the second segment.  Slower rates of advance are advisable.

One of the advantageous methods of scheduling for dropouts is to put the heat into the glass steadily.  I suggest there are two problems with the rates of advance and soaks in the above (partial) schedule.

The soak at 520°C would be more useful if it were at around 600°C.  This would allow the heat to be distributed throughout the glass before it begins to droop significantly.

The rapid advance of 330°C is much faster than needed, or desirable.  This rapid rate of advance allows the glass to move into the aperture, before the rim is plastic enough to stay on the supporting ring.

These rough drawings show how the rim initially rises from the ring, pivoting on the edge of the aperture.  This happens on all moulds (drops or slumps) where there is a rim.

With a rapid rise in temperature the raised rim edge gets more heat than the depressed middle, as it is closer to the elements.  This additional heat allows the edge of the rim to curve downwards forming air pockets as the edge touches back to the supporting ring.

Some people use fibre paper between the ring and the glass to prevent bubbles. This addition allows a passage of air from under the glass and reduces bubble formation.

Others have developed sloped drop out rings that eliminate the rising of the glass from a flat ring.  The glass is suspended above the aperture and only touches the edge of it as the glass softens. These crude drawings show the process.

To be certain of avoiding air bubbles under the rim of dropouts whatever the style of ring, you should use moderate rates of advance, with a possible soak at around 600°C which is before the glass begins any significant movement. The moderate rate of heating should be continued after this soak, rather than increased.

Drying kiln wash

“Dry your kiln wash between coats and before firing.” 

This is a frequent statement when talking about renewing kiln wash on shelves and moulds.  The main reason given seems to be that there will be less risk of creating bubbles by evaporating moisture.  The air drying will reduce moisture in kiln is a second reason.

There are some difficulties with this statement and reasons.

Drying between coats of kiln wash means you are applying liquid over powder. This can promote clumping and streaking through a too rapid absorption of water by the dry kiln wash. Also, it makes applying kiln wash a lengthy process.  It is not like painting a door or even a floor, where you must allow drying to avoid streaks. 

Credit: Ceramicartsnetwork.org

Applying kiln wash by brushing is smoothest if all coats are done at once.  This is what happens if you spray kiln wash on your shelves and that gives a smooth surface.  If it were otherwise, drying between coats would apply to spraying too.  Drying between coats promotes streaks in the applied kiln wash that needs to be smoothed before use.  This of course, does need to be done after the kiln wash has dried.

Drying before using the shelf or mould is unnecessary. The evidence I have to offer is that I frequently fire within an hour of applying fresh kiln wash to a cleaned shelf. I have had no problems with creating bubbles or glass picking up the kiln wash. The shelf dries, with a moderate rate of advance, long before the glass settles into the texture of the surface.  It is only as the glass settles into the contours of the kiln wash that it seals air, or any other material, under the glass.

The pigment in most kiln washes is to tell you which shelves have not yet been used.  If they are fired dry at even moderate temperatures, the pigment disappears.  Then you have removed that indicator of freshly prepared shelves or moulds.

Drying of kiln wash before use in not necessary.  If you wish to be cautious, air drying will be enough to avoid any problems with moisture.

What are enamels?

Not all enamels are equal

Enamel paints

This description refers to a paint that air dries (or with minimal heat) to a hard finish (usually gloss). Most commercially available enamel paints are significantly softer than either vitreous enamel or heat cured synthetic resins. The term "enamel paint" generally is used to describe oil-based covering products, usually with a significant gloss finish. Many latex or water-based paints have adopted the term.

Enamel paint has come to mean a "hard surfaced paint" and usually is in reference to paint brands of higher quality, floor coatings of a high gloss finish. Most enamel paints are alkyd resin based. Some enamel paints have been made by adding varnish to oil-based paint. Enamels paint can also refer to nitro-cellulose based paints. Nitro-cellulose enamels are also commonly known as modern lacquers.  These have been largely replaced by synthetic coatings like alkyd, acrylic and vinyl.

More information at: https://en.wikipedia.org/wiki/Enamel_paint

Enamel paints are used for coating surfaces that are outdoors or otherwise subject to hard wear, or variations in temperature.  A widespread application is in paints for cars. It is also used frequently to decorate or label bottles due to the low curing temperatures of some formulations.

Vitreous enamels 

Vitreous enamels are used in a variety of circumstances.  Metal signs are most frequently enamel coated; they are used in ceramics as over glazes;  and they are used on glass in many circumstances.

Vitreous Enamel is simply a thin layer of glass fused at high temperature on to the surface of a metal or glass. Vitreous Enamel can be defined as a material which is a vitreous solid obtained by smelting or fritting a mixture of inorganic materials.  The word enamel comes from the High German word ‘smelzan’ and from the Old French ‘esmail’.

The key ingredient of vitreous enamel is finely ground glass frit. Colour in enamel is obtained by the addition of various minerals and metal oxides. 

Vitreous enamel is made by smelting naturally occurring minerals, such as sand, feldspar, borax, soda ash, and sodium fluoride at temperatures between 1200°C and 1350°C  until all the raw materials have dissolved. The molten glass which is formed is either quenched into water or through water-cooled rollers. This rapid cooling prevents crystallisation. The resulting frit is ground in a rotating ball mill either to produce a water-based slurry or a powder.

At the milling stage, other minerals are added to give the properties and colour required of the final enamel. Different enamel colours can be mixed to make a new colour, in the manner of paint. Enamel can be transparent, opaque or opalescent.

More information at: 

https://www.vea.org.uk/what-is-enamel/

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.iom3.org/vitreous-enamellers-society/vitreous-enamel-information

Metal enamelling

Modern frit for enamelling steel is typically an alkali borosilicate glass with a thermal expansion and glass temperature suitable for coating steel and other metals. Raw materials are smelted together between 1,150 and 1,450°C (2,100 and 2,650°F) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit. Vitreous enamel is often applied as a powder or paste and then fired at high temperature. This process gives vitreous enamel its unique combination of properties. The smooth glass-like surface is hard; it is scratch, chemical and fire resistant. It is easy to clean and hygienic.  It all started 3500 years ago in Cyprus. Since 1500 BC, enamelling has been a durable, attractive and reliable material.

More information at: 

https://en.wikipedia.org/wiki/Vitreous_enamel

https://www.vea.org.uk/what-is-enamel/ 

Enamels in Ceramics

Overglaze decoration, overglaze enamelling or on-glaze decoration are all names for the method of decorating pottery, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature.  The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a varied and vivid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the clay body.

More information at:  https://en.wikipedia.org/wiki/Overglaze_decoration

Glass Enamels

Glass enamels are produced in the same way as enamels for metals and ceramics.  The frit characteristics are adjusted for various applications and temperatures.  This combination of finely ground frit and metals for colouring are often combined with a binder or carrier medium.  It is similar to vitreous enamel on metal surfaces, but the supporting surface is glass. It is also close to "enamelled" overglaze decoration on pottery, especially on porcelain, and it is thought likely that the technique passed from metal to glass (probably in the Islamic world), and then in the Renaissance from glass to pottery (perhaps in Bohemia or Germany). 

Glass may be enamelled by sprinkling a loose powder on a flat surface, painting or printing a slurry, or painting or stamping a binder and then sprinkling it with powder, which will adhere.  The powdered frit can be in the ceramic on-glaze composition suitable for fusing or casting temperatures, or it can be adjusted for slumping temperatures as in the traditional glass stainers’ enamels. It can produce brilliant and long-lasting colours, and be transparent, translucent or opaque. Generally, the desired colours only appear when the piece is fired, adding to the artist's difficulties.

More information at:  https://en.wikipedia.org/wiki/Enamelled_glass

The term enamel is applied to a wide variety of coating materials.  The range of usage is indicated, and the manufacture and applications of vitreous enamels is indicated.  The term enamel is not properly applied to finely ground coloured glass in a medium.